This invention relates to radio communication and particularly to cellular radio telephone systems that use time division duplex communication.
Applications of mobile and cordless radio telephony are becoming increasingly widespread, and cellular radio telephone systems are well known and have reached a high level of maturity. Cellular systems typically consist of a backbone network of a plurality of radio base stations located at strategic positions. Each base station covers a respective geographic area called a cell, and since adjacent cells partly overlap, a portable device like a mobile telephone can move from one cell to another without losing contact with the backbone network. As a portable device moves during a communication session, the connection is handed off from one radio base station to another according to a process that depends on, among other factors, the location of the portable relative to the base stations.
All over the world, cellular systems continue to be deployed to offer nation-wide public telephony. Current examples of wide-area mobile telephone systems are the Global System for Mobile communications (GSM), the Digital Advanced Mobile Phone System (D-AMPS), the IS-95 (CDMA) system, and the Personal Digital Cellular (PDC) system. These systems are run by operators that offer various public services using portions of the radio spectrum that typically are licensed by national regulatory bodies.
In addition to these licensed-spectrum cellular communication systems, new kinds of cellular system are now entering the market that are deployed in restricted areas like indoor environments (e.g., offices, houses, exhibition halls, etc.) and local areas (e.g., school campuses, office parks, etc.). These new systems are privately owned and typically use unlicensed portions of the radio spectrum like the globally available industrial, scientific, and medical (ISM) bands at 900 MHz, 2400 MHz, and 5700 MHz. Examples of such local-area, unlicensed-spectrum mobile communication systems are the Digital European Cordless Telephone (DECT) system, the Personal Handyphone System (PHS), and wireless local area computer networks (WLANs).
Adroit resource allocation in a cellular system is critically important. To connect a mobile or cordless terminal to the backbone network, both an access point to the network (e.g., a radio base station) must be available and a radio channel must be available to connect the terminal to the access point. Both the access point and the radio channel can be considered allocable system resources. When a connection has to be made, the access point and/or the terminal has to select a radio channel, but radio resources are scarce. The cellular system concept is a way to support a large number of terminals with a limited radio spectrum by organizing the spectrum into channels that can be used simultaneously for different connections, provided the geographical distance between users participating in different connections is large enough that their mutual interference is small relative to their intended received signals.
In most cellular systems, the access point closest to a remote terminal seeking a connection is allocated to that terminal since that access point usually provides the lowest propagation loss to that terminal. The remote terminals regularly scan the spectrum for control or beacon signals broadcast by the access points on predetermined radio channels, and each terminal locks, or synchronizes, itself to the strongest control or beacon channel it receives.
In some mobile systems, a terminal does not by default lock to the strongest access point but chooses an access point based on other criteria, e.g., whether a base station has radio channels available and/or whether the interference on any available radio channels is sufficiently low. Indeed, it is not the channel having the highest carrier power that is important but the channel having the highest carrier-to-interference (C/I) ratio. An exemplary communication system in which base station and channel selection are based on the C/I ration is described in the U.S. Pat. No. 5,491,837 to Haartsen for “Method and System for Channel Allocation Using Power Control and Mobile-Assisted Handover Measurements”, which is expressly incorporated here by reference.
In general, a “channel” can be a carrier frequency, a time slot, a code, or a hybrid of these, according to the particular access technique used by the communication system. In a frequency division multiple access (FDMA) system, a radio channel is a radio frequency (RF) carrier signal for transmitting and an RF carrier signal for receiving that are usually allocated for the duration of a communication session. (Separating the transmit and receive carriers, which are usually selected from respective dedicated bands, permits simultaneous transmission and reception and is called Frequency Division Duplex (FDD).) The Advanced Mobile Phone System (AMPS) and the Nordic Mobile Telephone (NMT) system are examples of simple FDMA systems that use carrier frequency modulation. In a time division multiple access (TDMA) system like GSM, each carrier signal is time-shared by up to eight users, i.e., each carrier signal transports successive frames of eight time slots each, and one or more time slots in each frame are allocated to the session. In a direct-sequence code division multiple access (CDMA) system, an information bit stream to be transmitted is effectively superimposed on a much-higher-rate bit stream that may consist of successive repetitions of a unique code sequence, and the superimposed bit streams may then be scrambled by multiplication by another, usually pseudo-noise, bit stream, with the result transmitted as a modulation of an RF carrier signal.
First-generation cellular systems like AMPS and the NMT system are analog, which is to say that an analog (temporally continuous) information signal to be transmitted modulates the frequency of the carrier signal. The primary use of analog systems is voice service, although low-rate digital data transmission is possible by using analog modems. In second-generation systems like GSM and D-AMPS, the information signal to be transmitted is digital (binary bits), which enables the information to be compressed, error-correction coded, organized into packets, and transmitted in bursts or packets. Thus, a carrier signal does not have to be in use all the time for one connection; instead, the carrier can be divided into slots, and different slots can be allocated to different users as in TDMA. In current second-generation TDMA systems, as well as in CDMA systems, the spectrum is still divided into bands of carrier frequencies, so such systems still have FDMA elements. If each carrier is divided into time slots, this results in a hybrid FDMA/TDMA system, and if each user is separated by a respective code, this results in a hybrid FDMA/CDMA system. Hybrid FDMA/TDMA/CDMA systems have also been described.
Another benefit of time-slotted systems is that downlink (base-station-to-remote-terminal) transmission and uplink (remote-terminal-to-base-station) transmission do not have to occur simultaneously, which is to say that FDD is not necessary. Instead, downlink and uplink transmissions can happen in different time slots on the same carrier, which may be called time division duplex (TDD). Full duplex operation is obtained by alternating between transmission and reception. Communication systems like cellular systems that are used for wide-area services still use FDD, which is preferable when access points are placed at elevated positions because FDD helps prevent interference between access points. Indoor communication systems and other high-data-rate systems preferably use TDD, in which the spectrum is not split into dedicated downlink and uplink bands. This enables the cost of the radio transceivers to be reduced because the transmission and reception processes occur sequentially with the same hardware, avoiding the costly duplexer that is otherwise required to provide sufficient isolation between the transmitter and receiver hardware. Moreover, downlink and uplink channels can be selected from an entire spectral band.
In conventional TDD systems like the DECT system, downlink and uplink channels use the same carrier frequency. More advanced TDD systems like the BLUETOOTH short range communication system use a frequency hopping concept: downlink transmission and uplink transmission occur in different time slots, but the carrier frequency for each slot can be different and can be any carrier in a particular radio band, irrespective of whether the slot is a downlink slot or an uplink slot. Information about the BLUETOOTH short range communication system is available on the Internet.
For symmetric services like voice communication, the bandwidths required in the downlink and uplink are substantially identical (both parties spend more or less equal time talking), and thus a division of the radio band into a downlink part and an uplink part which are of substantially equal size is reasonable. Data services, however, are typically asymmetric, which is to say that the bandwidths required in the downlink and uplink are substantially unequal (one party spends more time downloading than the other), and thus equal-sized downlink and uplink parts are less attractive. Some services, like Internet service, require much more downlink bandwidth, or capacity, than uplink bandwidth or the other way around. In the former case, the downlink band should preferably be larger.
In an FDD system, the downlink and uplink bands are usually fixed and cannot be changed, i.e., there is a strict separation between the downlink and uplink frequencies. A TDD system can allocate downlink and uplink channels much more flexibly since the downlink and uplink are not strictly separated; indeed, the entire band can be allocated temporarily for downlink or uplink services exclusively. In a TDD system as noted above, the channel is divided into a succession of time slots, and during each slot, one or more packets or bursts are transmitted. For a symmetric link, the distribution of slots between downlink and uplink transmission is substantially identical, and for an asymmetric link, the downlink, for example, can be allocated more slots than the uplink. This allocation can be carried out dynamically, i.e., as the traffic demand changes, the bandwidth distribution between downlink and uplink can be changed by changing the slot allocation.
Nevertheless, even the flexibility of TDD slot allocation has restrictions. A TDD radio transceiver can either transmit or receive, but not simultaneously. For a base station having a single transceiver, this is not a disadvantage but rather an advantage since expensive duplexers can be avoided, but this poses a problem if the base station has to be equipped with additional transceivers to increase capacity. Although duplexers are not an issue when transmission and reception happen in different radios, simultaneous transmission and reception is impossible if the transmit and receive frequencies are in the same band and the transmitter and receiver are physically close, as is the case when multiple radios have to be merged into a single base station, because when a transmitter transmits, its relatively high-level signal will saturate the front ends of the co-located receivers. The difference in level between a signal received from a co-located transmitter and a signal received from a remote terminal can be 70 dB or greater, which is so large that a base station receiver has no possibility of successfully filtering the desired signal from the remote terminal from the interfering signal from the co-located transmitter.
The only previously known way that a plurality of TDD transceivers can be integrated into the same base station is to synchronize their downlink and uplink transmissions such that either all transceivers transmit or all transceivers receive at the same time. This prevents the receivers from blinding by one or more of the transmitters, and is not much of a problem for a system providing symmetric services. Synchronized downlink and uplink transmissions simply means that slots must be allocated such that there is alignment between the downlink slots of different radios and alignment between the uplink slots of different radios. Asymmetric services, however, are severely hampered by a downlink/uplink synchronization requirement as it requires that the asymmetry (i.e., the imbalance between downlink and uplink bandwidths) is identical in all radios.
It is therefore an object of this invention to solve these problems in prior TDD communication systems and to provide methods and devices that support asymmetric services in a TDD system having multi-radio units and that use smart resource allocation and smart control of asymmetric traffic.